Seismic Information

The Size of an Earthquake     

     Basically, there exist three terms to describe the size of an earthquake: intensity, magnitude and acceleration. The intensity of an earthquake is the apparent degree of shock that is felt at different places. The intensity is determined on a particular site, writing down the effects that produce the shock in objects, buildings, people and in the land itself. The intensity scale that is used generally, especially in the Western Hemishpere, is called Modified Intensity Scale of Mercalli. This scale goes from I (not felt) to XII (total destruction) and was prepared by Charles Richter in 1956. The first scale of intensity was developed by Rossi, of Italy, and Forel from Switzerland in 1880, and was the one that was used to describe the effects of the earthquake of 1918 in Puerto Rico.

     The magnitude of an earthquake is determined taking the logarithm (base 10) from the strongest movement of the ground registered during the arrival of a type of seismic wave and applying the standard correction by the distance. Since the scale is logarithmic, the magnitude increases in a unit with the increase of ten units in the duration amplitude of the log of the seismic wave. Nevertheless, as far as energy released by an earthquake, an increase of one unit in the magnitude increases the amount of energy released in a factor of approximately 30. Although different scales of magnitude exist, based on different waves, most of them are reported in the Richter Scale in honor to the Dr Charles F. Richter who developed the concept in 1935.

     The size of an earthquake is also expressed in the acceleration due to the gravity, which is the acceleration with which a ball left in vacuum falls (1.0 g, where g is equal to 980 cm/s²). For the design of earthquake-resistant buildings, in addition to the acceleration it is also important to know the speed and displacement the ground, the duration of the earthquake and the properties of the waves. We next presente a comparative table of the values of intensity, magnitude and acceleration.

I II III IV V VI VII VIII IX X

I  Not felt. II. Only felt by people while they were resting, especially if tall buildings. Hanging objects oscillate a little. III Felt on the interior region. A lot of people do not recognize it as an earthquake. Parked automobiles shake. Vibrations similar to those felt by a moving truck. Noticeable duration. IV Felt inside structures by many people; only a few people feel it outside of structures. Windows, dishes and doors shake. Walls crack. Vibrations similar to those felt by a big moving truck; people get a shaking sensation. Parked vehicles can be seen moving from side to side. V Felt by almost everybody; a lot of people wake up. Some dishes, windows, etc. break; some rubble masonry houses crack. Unstable objects are turned upside down. Clock pentulums stop. Doors open and close by themselves. Trees and bushes can be seen shaking. VI These earthquakes are felt by everybody; a lot of people get scared and run outside. It becomes difficult to walk. Windows, dishes, and other glass object are broken. Some heavy furniture might be shaken; some rubblework may fall down; chimneys receive some damage. Slight damages in general. VI Everybody runs out of their home. Little damage suffered by well-designed buildings; slight to moderate damage to good structures; considerable damge to not so well built sturctures; some chimneys break. These earthquakes are felt by construction workers. VIII Slight damage to structures specially designed against earthquakes; considerable damage or even partial collapse of normal buildings; the damage is greater in poorly constructed structures.  Wall panels are torn out of their frames. Chimneys fall, as well as monuments, columns and walls. Heavy furniture could be turned upside down. Sand and mud landslides may occur. Rivers my grow in volume unexpectedly. It is hard to drive. IX Considerable damage in well-designed structures, good structures might be displaced from their foundations; the damage is even worse in regular buildings where total or partial collapse of the building may occur. Huge cracks on the ground.   Sand and mud can be seen running in alluvial areas. Subterranean pipes are damaged. X Some well-built structures made out of wood and some bridges are destroyed, most of the constructions and structure frameworks are destroyed along with their foundations. Huge cracks on the ground. Landslides are formed; rivers and lakes grow out of their banks. Both sand and mud are displaced horizontally. XI Most cement and concrete structures collapse. Bridges and other transportation routes are severely affected. XII Total loss of the infrastructure. Huge chunks of rocks are displaced. Heavy objects are thrown in to the air easily.

< 2.3 2.3 - 2.9 3.0 - 4.1 3.7 - 4.2 4.3 - 4.9 5.0 - 5.6 5.7 - 6.2 6.3 - 6.9 7.0 - 7.6 7.7 - 8.2 8.3 - 9.0 > 9.0

< 0.002 0.002 - 0.003 0.004 - 0.007 0.015 - 0.02 0.03 - 0.04 0.06 - 0.07 0.1 - 0.15 0.25 - 0.3 0.5 - 0.55 > 0.6

References:

• Bolt, B., 1981.  Terremotos.  Editorial Reverté, Barcelona, 266p.
• United States Atomic Energy Commission.  Reactores Nucleares y Terremotos, TID-7024.